The charismatic Bishop Ambrose (Ambrogio) - who defended orthodox Christianity against Arianism and later became Milan's patron saint - had this Basilica Martyrum built between 379 and 386. The remains of local martyr-saints Gervasius and Protasius still lie in the crypt.The church was enlarged in the eighth century, when the Benedictines erected the Campanile dei Monaci (monks' bell tower) to the right of the façade. In the ninth century, under Archbishop Anspert, the atrium preceding the façade was added; it was here that the populace sought sanctuary in times of trouble. The church's Romanesque appearance stems from ninth- and tenth-century redesigns. Anspert's atrium was remodelled in the 11th century, when a reconstruction of the church got under way. Its capitals feature biblical scenes and mythical beasts symbolising the struggle between Good and Evil.

The Torre dei Canonici ('canons' tower') to the left of the façade was built between 1128 and 1144. Further changes to the interior of the church were made in 1196 after the dome collapsed.

In 1492 Ludovico 'il Moro' Sforza called on Donato Bramante to remodel the eighth-century Benedictine monastery. The fall of il Moro in 1499 put an end to Bramante's makeover, which as a result was limi-ted to one side of the old cloister (the Portico della Canonica, accessible from the left of the nave). The church had a lucky escape from a remodelling job in the 17th century, but suffered severe air-raid damage in 1943; the bombing destroyed Bramante's work, which was subsequently rebuilt using salvaged original materials.

The interior has the sober proportions of the austere Lombard Romanesque style, its three aisles covered with ribbed cross-vaults and false galleries holding up the massive walls. Beneath the pre-Romanesque pulpit, reconstructed from the original pieces after the dome collapsed on it in 1196, lies what is known as the Stilicone Sarcophagus. This fourth-century masterpiece was traditionally believed to have been the burial place of the Roman general Stilicone, who served the Emperor Honorius and died in 408; later research disproved this legend. The 12th-century golden altar, illustrated with scenes from the life of Christ on the front and of St Ambrose on the back, once covered the porphyry casket commissioned to house the remains of Ambrose, Protasius and Gervasius when they were dug up in the ninth century.

To the right of the main altar, a series of chapels leads to the Sacello di San Vittore in Ciel d'Oro. Part of the church's original fourth-century structure, this chapel was clinically reworked in the 1930s, so that only its glowing, golden fifth-century mosaics in the dome remain to remind us of its antique glory. They portray St Ambrose standing between Gervasius and Protasius, with a sprinkling of other minor local martyrs looking on. This section of the church has been converted into a small museum (access is through the 18th-century Cappella di Sant'Ambrogio Morente) consisting of the mosaics and precious church furnishings.

The museum that was once housed in the cloisters has been split up and the exhibits moved to the Museo Diocesano and the San Vittore in Ciel d'Oro chapel here. There's also an exhibition space, albeit normally for modern art, in the Antico Oratorio della Passione (piazza Sant'Ambrogio 23A, opening times vary, admission free).